With FETs, in particular MOS FETs, the body diode in inverse operation which is present because of the technology often causes problems in practice. This body diode, which is parallel to the source drain path, is polarized in the reverse direction in operation. With inductive loads, a commutation current or commutation voltage that occurs when the current is stopped or reversed may flow across the body diode. With the conventional MOS FETs, for example, the conducting-state voltage of the body diode in inverse operation is 0.8 to 1.2 V. The reverse recovery time depends on the model, amounting to about 150 ns with 50 V semiconductors and increasing to about 2 .mu.s at a higher reverse voltage.
Since semiconductor switches like MOS FETs are very fast switches, there are those who would also like to use them at high switching rates. With inductive loads such as transformers, throttles and motors, high losses occur due to stored charges at high switching rates in the body diode in inverse operation.
The losses that often occur at high switching rates in practice have led to various proposals to remedy the situation:
By special heavy metal doping of the FET body diode, the stored charges with FREDFETs can be minimized without affecting the other parameters of the semiconductor switch (FREDFET=fast-recovery epitaxial diode field effect transistor). However, the losses caused by the stored charges that nevertheless remain are so high at high switching rates that they interfere with switching performance. In addition, the conducting state power losses of the body diode increase with a reduction in stored charge.
According to another conventional method, an external free-wheeling diode is used to bridge a series connection of the FET and a diode polarized in the conducting direction in the working direction of the FET. Thus, the path across the semiconductor switch is blocked in inverse operation by the diode arranged in series and a current path is provided through the bridging diode. This assures that in inverse operation no current can flow across the semiconductor switch and thus its body diode. Very fast diodes such as Schottky diodes are used for such diodes. Losses due to a stored charge are thus greatly reduced, but disadvantages include the increased cost of the additional special diodes and especially the switching losses in these diodes.
The object of the present invention is to develop an electronic switch of the type defined in the preamble that will be suitable for high switching rates even with inductive loads without having the disadvantages that were previously unavoidable.